Initiating in situ combustion in a stratum



April 7, 1959 H. w. PARKER 2,880,803

INITIATING IN SITU COMBUSTION IN A STRATUM Filed Jan. 16, 1958 AIR FUEL GAS FIRE FRONT FRODUCING FAIR STRATUM /FUE|. GAS

FIG. 3

"FUEL GAS INVENTORQ \FIRE FRONT IHLW- PARKER BY W 1% ATTORNEYS United States PatentO INITIATING IN SITU COMBUSTION IN A STRATUM Harry W. Parker, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application January 16, 1958, Serial No. 709,372 13 "Claims. (Cl. 166--11) This invention relates to a process for initiating in situ combustion in a carbonaceous stratum and to the recovery of hydrocarbons therefrom.

Insitu combustion in the recovery of hydrocarbons from underground strata containing carbonaceous material is becoming more prevalent in the petroleum industry. In this technique of production, combustion is initiated in the carbonaceous stratum and the resulting combustion zone is caused to move thru the stratum by either inverse or direct air drive whereby the heat of combustion of a substantial proportion of the hydrocarr bon in the stratum drives out and usually upgrades a substantial proportion of the remaining hydrocarbon material. The ignition of carbonaceous material in a straturn around a borehole therein followed by injection of air thru the ignition borehole and recovery of product hydrocarbons and combustion gas thru another borehole in the stratum is a direct air drive process for effecting in situ combustion and recovery of hydrocarbons from the stratum. In this type of operation the stratum usually plugs in front of the combustion zone because a heavy viscous fluid bank collects in the stratum in advance of the combustion zone which prevents movement of air to the combustion process. To overcome this difliculty and permit the continued progress of the combustion zone thru the stratum, inverse air injection has been resorted to. By this technique, a combustion zone is established around an ignition borehole by any suitable means and air is fed thru the stratum to the combustion zone from one or more surrounding boreholes.

In either type of in situ combustion, it is necessary to initiate combustion in the carbonaceous stratum before the combustion zone can be moved thru the stratum. One method of initiating combustion in the stratum comprises heating the borehole around which ignition is to be effected by means of a downhole heater either of the gas type or electric.

Patented Apr.- 7, 1959 injection equipment. It is also an object of the invention to provide a process for igniting a carbonaceous stratum which requires no preheating of the ignition borehole. Other objects of the invention will become apparent upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises separately injecting a suitable fuel gas and a combustion-supporting, free-oxygen-containing gas into a gas-permeable carbonaceous stratum and passing the gases to an ignition borehole, whereby the gases mix along an interface leading to the ignition borehole, and igniting the mixture of gases in the ignition borehole as the mixture enters thesame so that combustion takes place in the pores of the formation and the combustion zone spreads along the interface thereby heating the stratum to a substantial depth from the borehole. After a substantial area of the stratum is thus heated to at least the ignition temperature of the carbonaceous material therein, the material is ignited by reducing the flow of fuel gas or by cutting off the same entirely so that free-oxygen contacts the heated stratum and thereby ignites the same. It is preferred to utilize oxygen or oxygen-enriched air as the combustion supporting gas since combustion progresses more readily into the stratum. and produces greater heating therein than with a lower concentration of oxygen. However, the process can be effected by injection of air without added oxygen.

In a preferred embodiment of the invention, fuel gas, such as propane, is injected into the selected carbonaceous stratum thru a plurality of injection boreholes in a ring pattern around a central ignition borehole and the combustion supporting gas is injected thru alternate boreholes in the ring pattern so that each stream of fuel gas is flanked by combustion supporting gas, and vice versa, as the gases pass toward the ignition borehole. As the fuel gas and oxygen enter the ignition borehole, the mixture is ignited by means of a spark, pyrotechnic, or other means, and the combustion penetrates the stratum be; cause of the mixing of the oxygen and fuel gas along an interface which follows generally radii extending out from the ignition borehole between each pair of fuel and oxygen injection boreholes. At the interface or boundary of the two gases, particularly when using oxygen or oxygen enriched air, there is an intense oxygengas flame which begins at the sand face in the ignition well and I propagates back into the formation along the line of the Field experience with this method of initiating combustion has not been encouraging because of the frequent damage of the downhole heater due to overheating thereof. Another method of initiating combustion involves injecting a premix of fuel gas and air into the stratum around the ignition borehole and igniting the same in the formation around the borehole. In this type of operation there is danger of explosion of the explosive mixture in the equipment and in the borehole leading to the stratum. The instant invention is concerned with a method and process for initiating combustion in a carbonaceous stratum which avoids the problems of prior art methods.

Accordingly it is an object of the invention to provide a novel and improved process for initiating in situ combustion in a permeable carbonaceous stratum. Another interface. This intense flame in the rock Vaporizes the tar or other carbonaceous material and provides fuel for the oxygen which spreads to a substantial area of the oxygen flow. The flame is self-sustaining using oxygen and therefore the flow of fuel-gas can be replaced with oxygen to effect in situ combustion of the carbonaceous material. The fire or combustion zone is thereby spread around the ignition well and the oxygen is then replaced with air to support combustion of the carbonaceous material in the stratum. The method of replacing fuel gas with air, and oxygen oroxygen-enriched air with air, preferably is a gradual process in order that damping of the burning flame or combustion zone does not occur.- Generally the period of replacement will be in the range of l to 24 hours.

The invention is readily applicable to tar sands but is also adaptable to oil sands, athabasca sands, shale, and coal, particularly, lignitic type coals which are naturally porous. Strata which are impermeable or nonporous, such as certain types of coal, may be made permeable by conventional fracturing methods. If fracturing is utilized it should be effected along a generally horizontal.

Fora more complete understanding of the invention reference is made to the accompanying schematic drawing of which Figure 1 is a plan view of a conventional S-spot Well pattern illustrating the invention; Figure 2 is an elevation along the line AA of Figure 1 thru a carbonaceous stratum showing an arrangement of boreholes and equipment for elfecting the invention; and Figure 3 is a plan view of an in-line well spotting for in line drive in accordance with the invention.

Referring to Figures 1 and 2 of the drawing, an ignition or production well 10 is positioned centrally with respect to a ring of injection wells 12, for fuel gas injection, and 14, for injection of combustion supporting gas such as air oxygen-enriched or substantially pure oxygen. Well 10 is equipped with a casing 16 and tubing 18. A line 20 connects with casing 16 below the well head for the withdrawal of gases from the well and/or the injection of fluids thereto, such as water, to prevent the combustion of produced hydrocarbons within the annulus and well bore. The gas injection wells 12 and 14 are provided with casing 22 and tubing 24 and 26, respectively. Well 10 is usually provided with production equipment (not shown in detail in the drawing) for the recovery of liquid and gaseous hydrocarbons from the well bore.

In operation of the process of the invention a suitable fuel gas such as propane is injected thru corner wells 12 and free-oxygen-containing gas, preferably oxygen-enriched air or pure oxygen, is injected thru alternate corner wells 14, center well 10 being open so that the injected gases flow to this well. The fuel gas and combustionsupporting gas mix along an interface represented by radii 28. As the mixed gases enter well 10 the mixture is ignited by a spark or other suitable means and the combustion which at first takes place on the wall of the borehole gradually migrates into the stratum substantially along radii 28 and the formation gradually becomes heated to a temperature above combustion-supporting temperature of the tar or other carbonaceous material in the stratum. During this time components of the carbonaceous material are fluidized and gasified so that they permeate the stratum in the combustion area surrounding borehole 10, thereby making it feasible to cut off the fuel gas injection rate so that the combustion feeds on the gasified hydrocarbon in the formation. As the fuel supply is cut off gradually, the same is replaced with combustion supporting gas, preferably of the same character as that injected thru boreholes 14. After a substantial heat reservoir has been established around borehole 10, the combustion supporting gas is replaced with air in instances where pure oxygen or oxygen-enriched air has been utilized to initiate the ignition. It'is, of course, to be understood that the process thus far described can be effected with air, though less readily and with greater time consumption. After the in situ combustion has been well established, injection of air thru boreholes 12 and 14 feeds the combustion and advances the combustion zone radially outwardly from borehole 10 toward the injection boreholes and eventually the combustion zone reaches these injection boreholes. When the combustion zone arrives' at the injection boreholes, continued injection of air causes a reversal in the direction of the combustion zone, driving it back by direct drive to production well 10. During this time the combustion is supported by carbonized residue left in the stratum during the inverse burning step.

It is also feasible to inject air or other combustion supporting gas into the stratum thru a ring of wells or boreholes (not shown) surrounding the -spot pattern shown in Figure 1, the injection commencing when the combustion front arrives at boreholes 12 and 14 with the injection thru these original injection boreholes-continuing so that the'cornbustion' zone is simultaneously rnov'ed outwardly from boreholes 12 and 14 by airinje'cted'from the outer ring of-boreholes and-inwardly toward borehole by direct-drive of air injected'thruboreholes Rand--14. It

is also feasible to terminate the injection of combustion supporting gas thru boreholes 12 and 14 when the cornbustion zone arrives at these wells and inject air or other combustion-supporting gas thru the outer ring of injection boreholes, only.

In accordance with another embodiment of the invention, ignition of the carbonaceous material in the stratum may be effected in a plurality of in-line ignition boreholes 10 flanked on either side by a line of injection boreholes with injection of fuel gas and air in alternate boreholes 12 and 14. This pattern of well spotting and operation is illustrated in Figure 3 wherein wells 10 are spaced on a line intermediate two lines in injection wells 12 and 14 alternately positioned in each line with wells 10 offset midway between lines connecting opposite wells. The stratum is ignited around boreholes 10 and the combustion advances along lines 28 in the same manner as in a 5-spot pattern, but the combustion zone moves outwardly in both directions from the line of holes 10.

In some cases it may be dsirable to preheat the borehole of the ignition or production well in order to reduce the oxygen requirements and thereby reduce the cost of operation. In this event, the wall of the borehole is heated to only mildly elevated temperatures such'as temperatures in the range of 300 to 450 or 500 F. without endangering overheating of the downhole heater. It is also feasible to efiect a preheat of the ignition borehole by pyrotechnics such as by the burning of a mass of self-sustaining solid fuel. A fuel of this type is that used in rocket propulsion or Jato fuels.

Other cycles of gas injection or well patterns may be utilized, the invention lying in the simultaneous injection of fuel gas and oxygen thru difierent wells so that a mixture' of these gases passes thru the stratum into a produc tion or ignition well where it is ignited by a spark or other suitable means thereby spreading combustion of the combustible mixture into the stratum so as to heat the same to combustion-supporting temperature of the carbonaceous material therein. This technique facilitates ready initiation of in situ combustion and recovery of hydrocarbons from the stratum by this technique. It is to be further understood that while it is preferable to utilize the invention with inverse air injection, my ignition initiation technique and establishment of in situ combustion may be followed by direct air injection thru the ignition borehole with one or more of the original injection boreholes being utilized as production boreholes.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details'disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. A process for initiating in situ combustion in a permeable carbonaceous stratum which comprises passing a fuel gas thru a section of said stratum into an ignition borehole therein; passing combustion-supporting, freeoxygen-containing gas thru an adjacent section of said stratum into said borehole whereby the oxygen and fuel gas mix along an interface of said gases and in said borehole; igniting the mixture of gases in said ignition borehole so as to burn said mixture in said borehole and cause same to burn within said stratum; after a substantial area of said stratum is heated to at least the ignition temperature of the carbonaceous material therein, terminating'the flow of fuel gas thereto and supplying free-oxygen-containing gas to the hot area so as to burn said carbonaceous material and effect in situ combustion.

2. The process of claim 1 wherein the concentration of oxygen in said combustion-supporting gas is higher than in air.

3; Theprocess of claim 1 wherein said combustionsupporting gas is air.

4. A process for initiating in situ combustion in a permeable carbonaceous stratum which comprises injecting a fuel gas into said stratum thru a plurality of injection boreholes spaced apart in a ring around an ignition borehole so as to pass said gas thru said stratum to said ignition borehole; injecting a combustion-supporting, oxygen-containing gas into said stratum thru a plurality of injection boreholes alternately spaced with respect to the fuel injection boreholes so that the two gases mix on an interface along radii from said ignition borehole passing between each pair of fuel gas and oxygen injection boreholes; igniting the mixture of gases entering said ignition borehole so as to burn said mixture therein and within said stratum; when a substantial area of said stratum is thereby heated to a temperature above combustion-supporting temperature replacing the fuel gas with oxygencontaining gas so as to ignite the carbonaceous material in said stratum and initiate in situ combustion therein.

5. The process of claim 4 wherein said combustionsupporting gas is air.

6. The process of claim 4 wherein said combustion supporting gas is richer in oxygen than air.

7. The process of claim 4 wherein air is injected thru said injection boreholes after initiation of in situ combustion so as to advance the combustion area thru said stratum to the proximity of said injection boreholes counter-currently to the flow of air.

8. The process of claim 4 wherein said combustionsupporting gas comprises oxygen-enriched air and said fuel gas and combustion-supporting gas are gradually replaced by air after in situ combustion is established so as to move the combustion zone thru said stratum toward said injection boreholes.

9. The process of claim 8 wherein produced hydrocarbons are recovered from said ignition borehole.

10. A process for initiating in situ combustion in a. permeable carbonaceous stratum which comprises injecting a free-oxygen-containing gas thru a plurality of first injection boreholes therein spaced in two rows, one on each side of a row of ignition boreholes in said stratum, so that oxygen passes to said ignition boreholes; injecting a fuel gas thru a plurality of second injection boreholes in said stratum in said rows, alternately posi tioned with respect to said first injection boreholes so that said fuel gas passes to said ignition boreholes and mixes with oxygen along an interface on radii extending from said ignition boreholes between the closest pair of first and second ignition boreholes; igniting the mixture of gases entering each ignition borehole so as to burn same therein and cause the combustion to move into the stratum around each ignition borehole; after a substantial area of said stratum around each borehole has been heated to combustion supporting temperature of the carbonaceous material therein, supply. free oxygen to the heated area so as to ignite said carbonaceous material and establish in situ combustion around said ignition boreholes.

11. The process of claim 10 wherein ignition of said carbonaceous material is initiated by terminating the injection of fuel gas and substituting the injection of air therefor.

12. The process of claim 11 wherein air is injected thru said first and second injection boreholes after establishing in situ combustion whereby the combustion zone is advanced toward each row of injection boreholes, and production is recovered thru said ignition boreholes.

13. The process of claim 1 in which the flow of fuel gas is terminated gradually.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS FOR INITATING IN SITU COMBUSTION IN A PERMEABLE CARBONACEOUS STRATUM WHICH COMPRISES PASSING A FUEL GAS THRU A SECTION OF SAID STRATUM INTO AN IGNITION BOREHOLE THEREIN; PASSING COMBUSTION-SUPPORTING, FREE OXYGEN-CONTAINING GAS THRU AN ADJACENT SECTION OF SAID STRATUM INTO SAID BOREHOLE WHEREBY OXYGEN AND FUEL GAS MIX ALONG AN INTERFACE OF SAID GASES AND IN SAID BOREHOLE; IGNITING THE MIXTURE OF GASES IN SAID BOREHOLE SO AS TO BURN SAID MIXTURE IN SAID BOREHOLE AND CAUSE SAME TO BURN WITHIN SAID STRATUM; AFTER A SUBSTANTIAL AREA OF SAID STRATUM IS HEATED TO AT LEAST THE IGNITION TEMPERA- 